chore (ai): remove useChat keepLastMessageOnError (#6114)

## Background

`keepLastMessageOnError` is deprecated and should be removed in AI SDK
5.

## Summary

Remove `keepLastMessageOnError` and simplify adjacent code.

Author: lgrammel

.changeset/funny-mayflies-yawn.md

@@ -0,0 +1,5 @@
+---
+'ai': major
+---
+
+chore (ai): remove useChat keepLastMessageOnError

packages/ai/core/types/ui-messages.ts

@@ -223,13 +223,6 @@ Additional data to be sent to the API endpoint.
 
 export type UseChatOptions = {
   /**
-Keeps the last message when an error happens. Defaults to `true`.
-
-@deprecated This option will be removed in the next major release.
-   */
-  keepLastMessageOnError?: boolean;
-
-  /**
    * The API endpoint that accepts a `{ messages: Message[] }` object and returns
    * a stream of tokens of the AI chat response. Defaults to `/api/chat`.
    */

packages/ai/core/util/call-chat-api.ts

@@ -1,7 +1,8 @@
+import { JSONValue } from '@ai-sdk/provider';
+import { IdGenerator, UIMessage, UseChatOptions } from '../types';
 import { processChatResponse } from './process-chat-response';
 import { processChatTextResponse } from './process-chat-text-response';
-import { IdGenerator, UIMessage, UseChatOptions } from '../types';
-import { JSONValue } from '@ai-sdk/provider';
+import { request } from 'http';
 
 // use function to allow for mocking in tests:
 const getOriginalFetch = () => fetch;
@@ -13,7 +14,6 @@ export async function callChatApi({
   credentials,
   headers,
   abortController,
-  restoreMessagesOnFailure,
   onResponse,
   onUpdate,
   onFinish,
@@ -30,7 +30,6 @@ export async function callChatApi({
   credentials: RequestCredentials | undefined;
   headers: HeadersInit | undefined;
   abortController: (() => AbortController | null) | undefined;
-  restoreMessagesOnFailure: () => void;
   onResponse: ((response: Response) => void | Promise<void>) | undefined;
   onUpdate: (options: {
     message: UIMessage;
@@ -45,9 +44,9 @@ export async function callChatApi({
   getCurrentDate: () => Date;
   requestType?: 'generate' | 'resume';
 }) {
-  const request =
+  const response =
     requestType === 'resume'
-      ? fetch(`${api}?chatId=${body.id}`, {
+      ? await fetch(`${api}?chatId=${body.id}`, {
           method: 'GET',
           headers: {
             'Content-Type': 'application/json',
@@ -56,7 +55,7 @@ export async function callChatApi({
           signal: abortController?.()?.signal,
           credentials,
         })
-      : fetch(api, {
+      : await fetch(api, {
           method: 'POST',
           body: JSON.stringify(body),
           headers: {
@@ -67,21 +66,11 @@ export async function callChatApi({
           credentials,
         });
 
-  const response = await request.catch(err => {
-    restoreMessagesOnFailure();
-    throw err;
-  });
-
-  if (onResponse) {
-    try {
-      await onResponse(response);
-    } catch (err) {
-      throw err;
-    }
+  if (onResponse != null) {
+    await onResponse(response);
   }
 
   if (!response.ok) {
-    restoreMessagesOnFailure();
     throw new Error(
       (await response.text()) ?? 'Failed to fetch the chat response.',
     );

packages/react/src/use-chat.ts

@@ -142,7 +142,6 @@ export function useChat({
   body,
   generateId = generateIdFunc,
   fetch,
-  keepLastMessageOnError = true,
   experimental_throttle: throttleWaitMs,
   ...options
 }: UseChatOptions & {
@@ -303,11 +302,6 @@ Default is undefined, which disables throttling.
             ...chatRequest.headers,
           },
           abortController: () => abortControllerRef.current,
-          restoreMessagesOnFailure() {
-            if (!keepLastMessageOnError) {
-              throttledMutate(previousMessages, false);
-            }
-          },
           onResponse,
           onUpdate({ message, data, replaceLastMessage }) {
             mutateStatus('streaming');
@@ -391,7 +385,6 @@ Default is undefined, which disables throttling.
       abortControllerRef,
       generateId,
       fetch,
-      keepLastMessageOnError,
       throttleWaitMs,
       chatId,
       getCurrentDate,

packages/svelte/src/chat.svelte.ts

@@ -23,7 +23,7 @@ import {
 } from './chat-context.svelte.js';
 
 export type ChatOptions = Readonly<
-  Omit<UseChatOptions, 'keepLastMessageOnError'> & {
+  UseChatOptions & {
     '~internal'?: {
       currentDate?: () => Date;
     };
@@ -266,7 +266,6 @@ export class Chat {
           ...chatRequest.headers,
         },
         abortController: () => abortController,
-        restoreMessagesOnFailure: () => {},
         onResponse: this.#options.onResponse,
         onUpdate: ({ message, data, replaceLastMessage }) => {
           this.#store.status = 'streaming';

packages/vue/src/use-chat.ts

@@ -125,7 +125,6 @@ export function useChat(
     generateId = generateIdFunc,
     onToolCall,
     fetch,
-    keepLastMessageOnError = true,
     maxSteps = 1,
     experimental_prepareRequestBody,
     ...options
@@ -259,12 +258,6 @@ export function useChat(
           }
         },
         onFinish,
-        restoreMessagesOnFailure() {
-          // Restore the previous messages if the request fails.
-          if (!keepLastMessageOnError) {
-            mutate(previousMessages);
-          }
-        },
         generateId,
         onToolCall,
         fetch,